The invention relates generally to printing machines and, more particularly, relates to a method of stipulating values for use in connection with the open-loop or closed-loop control of a printing machine.
In the production of printed products it is known to register the application of ink quantitatively on measurement elements which are additionally printed at the same time or at measurement points in the actual printed image. Widespread use is made of the so-called color density measurement, in which measurement points having one or more printing inks are measured using appropriate filters in order to determine the ink film thickness. It is likewise known to measure measurement points, i.e., printing control strips or measurement points in the image, calorimetrically, in order, via the actual color loci thus obtained in conjunction with prescribed, desired color loci, including appropriate color offset (vector) formation, to derive the necessary actuating variables for the correction of the printing process. Furthermore, spectral measuring methods are known in which reflectances are determined over a multiplicity of reference points or continuously, for example, within the visible spectrum. In this case, the registered actual spectrum of a sample point of the printed product is compared with a prescribed desired spectrum, for example, a sample sheet or original, in order to derive therefrom the appropriate actuating commands for the ink feed. Spectral measurement and control methods of this type are typically used in gravure printing, since, in the case of this printing process, the additions to be fed to a color for the purpose of reaching a desired reflectance can be determined in a simple way from a registered spectral difference.
By way of example, DE 4 311 132 A1 discloses a method for effectuating closed-loop/open-loop control in a printing machine. In the disclosed method, the density spectra of sample prints of the individual printing inks with a stipulated area coverage, as well as the paper whiteness, are stored, the density spectra of a least one measurement point per inking zone on an original and corresponding measurement points on the printed product are determined, and the measured density spectra of the original and the printed product are represented as a linear combination of the density spectra weighted with the degrees of area coverage of the sample prints of the individual printing inks and of the paper whiteness. Subsequently, the degrees of area coverage are calculated in such a way that the density spectra are approximated in an optimum manner by the linear combination. In this manner, the ink feed is changed, in the event of a deviation of the degrees of area coverage of printed product and original, until an equalization of the density spectra is achieved. What is disadvantageous in the case of this method is that the factors in the formulation of the linear combination in order to represent the overall density spectrum, i.e., the combined print of the image points, are formulated from the density spectra of the individual colors as degrees of area coverage. A formulation of this type with respect to the factors presents problems at high degrees of area coverage of a color at an image point and ultimately fails during the printing of one or more colors in full-tone. The reason for this is that if, for example, one color of the original is printed as a full-tone area, the corresponding color in the printed copy can likewise be printed only as a full-tone and, moreover, not more intensely. Furthermore, this method presupposes that there already exists an original which was produced exactly with the printing inks used in the subsequent print, and on which the spectral reflectances are then determined at the corresponding image points.
By way of further example, DE 4 343 905 C2 discloses a method of determining the necessary change in the ink feed in a multicolor printing machine, in particular, an offset printing machine. In this method, the color density spectra of at least one colored image point and, simultaneously, the individual printing inks involved in the color at the point are recorded with regard to an original copy. In the case of each following printed copies, the color density spectra of the original and the printed copy are represented as a linear combination of the color density spectra of the individual colors, which are weighted with degrees of area coverage and paper whiteness. The degrees of area coverage of the individual colors are determined by linear regression. Based upon a comparison of the color density spectra, the derivation of an ink feed adjustment is performed in order to equalize the inking of the product copy to that of the original copy.
The comparison is accomplished by a differential color density spectrum which is determined from the color density spectra of corresponding image points of the original and of the printed copy and, in addition, the color density spectra of the printing inks (colored inks+black) involved in the print of the colored image. The differential color density spectrum between the original and printed copy is then represented as a linear combination of the color density spectra of the printing inks (colored inks+black) involved in the print of the printed copy, the coefficients of this linear combination being determined with the aid of linear regression. The coefficients which are determined from the linear combination then give a measure as to how the individual colors (colored inks+black) involved in the print of the printed copy must be changed in terms of their proportions. In the case of this method, it is irrelevant whether the original has been printed with the printing inks used later, or whether the original has even been produced using a quite different reproduction process. This color control method also operates using theoretically stipulated desired color density spectra.
Further methods disclosed in DE 4311 132 A1 and DE 4 343 905 C2 require that the desired color density spectra be determined at a multiplicity of image points of an original. A multiplicity of the printed products produced is set up as an original on image processing computers. Known art additionally permits the image data obtained in this manner, following appropriate color separation and subsequent raster image processing, to be further processed directly in order to generate the necessary printing forms, i.e., printing plates in offset printing. In the case of a purely digital workflow between the generation of the printing original and of the printing forms/printing plates to be used for printing, it is possible to dispense entirely with the production of originals in the form of sample prints, proofs or the like. In this case, it is even possible to stipulate desired colored loci of the digital original in the strictly calorimetric sense, since, by using color management systems, the characteristics of the color change between the output devices and the monitor of an image processing station/printing machine can be taken into account.
In DE 4 343 905 C2 yet another method for determining the necessary change of the ink feed in a multicolor printing machine is disclosed which has decisive advantages with regard to the convergence of the actual color density spectra of the printed copies which are to be guided to the desired color density spectra. However, this process presupposes previously known desired color density spectra and is in no way integrated into the color management systems which are known at present since color loci, i.e., desired color locus, actual color locus, are not used in this method.
EP 0 142 470 B1 discloses still a further method and device for assessing the print quality of a printed product which is preferably produced on an offset printing machine. In order to assess the print quality, the printed product and an original are each divided into image elements and measured photoelectrically image element by image element. The desired and actual reflectance values of each individual printing ink are registered densitometrically and use is also made of a sensation weighting factor specifying the relation between subjectively measured and metrological color deviation, given appropriate reflectance values. However, this method offers no possibility of deriving desired color density spectra from a printed original, in particular, one which is present in digital form, which spectra can then be used in particular in a method such as disclosed in DE 4 343 905 C2 for the production of printed products.
Likewise, EP 0 142 469 B1 discloses a method for the closed-loop control of the ink feed in an offset printing machine in which, at least during the start-up phase of the printing machine, in order to control the initial print of the individual printing inks, a reference in the form of the respective printing plate or a photographic original on which it is based is divided into a multiplicity of image elements and for each image element the area coverage is determined. From this, reflectance desired values are formed which are compared with reflectance values obtained image element by image element on the printing elements and are used for the derivation of actuating commands for the ink feed. As in the case of the method disclosed in EP 0 142 470 B1, this procedure also does not supply any desired color density spectra which would then be used for the control of the ink feed.
It is, therefore, a general object of the present invention to provide a method of stipulating values for use in the control of a printing machine. More particularly, it is an object of the invention to provide such a method that avoids the above-mentioned disadvantages while enabling a determination of desired spectra or desired color density spectra of image points of an original, in particular, a digital original.
According to the invention, it is provided that, in a digitally produced image original, a specification of the corresponding measurement fields is performed to the effect that, using these measurement fields, the subsequent print is also controlled, open-loop or closed-loop, during the production of the individual printed products. During the open-loop or closed-loop control of the print, use is preferably made of a method which, as explained above, determines the difference between a desired and an actual density spectrum and represents the so-called differential color density spectrum as a linear combination of the printing inks (colored inks+black+special inks) used in the print. By means of the area coverage of the individual colors, which can be derived from the digital image original, the spectra of the colors to be used during printing and the desired color loci which can be assigned to the image points, desired spectra or desired color density spectra are then determined on the basis of a model simulating the combined print.
In order to determine the desired color density spectrum a desired spectrum for a measurement field of the digital original is first determined. This spectrum is then used, if appropriate following conversion by taking logarithms into a desired colored density spectrum, for the open-loop/closed-loop control in accordance with the above-mentioned method.
A better understanding of the objects, advantages, features, properties and relationships of the invention will be obtained from the following detailed description and accompanying drawings which set forth an illustrative embodiment and are indicative of the various ways in which the principles of the invention may be employed.